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United States Patent |
6,124,646
|
Artinian
,   et al.
|
September 26, 2000
|
Aircraft air conditioning system including electric generator for
providing AC power having limited frequency range
Abstract
An air conditioning system for an aircraft includes cascaded high and low
pressure turbines, an electric generator that is driven at variable speeds
by the low pressure turbine, and a fan that is also driven by the low
pressure turbine. During operation of the air conditioning system, ac
power is generated by the electric generator and provided to electrical
equipment onboard the aircraft. Because the electric generator is driven
at variable speeds, it generates ac power having variable frequency. The
air conditioning system further includes a bypass valve for bypassing the
high pressure turbine when speed of the electric generator approaches a
lower limit. Bypassing the high pressure turbine allows generator speed to
be increased. The fan has a speed-cubed load characteristic that maintains
the generator speed below an upper limit. Thus, the bypass valve and the
fan can be used to limit the speed of the electric generator and thereby
maintain frequency of the ac power between upper and lower limits.
Inventors:
|
Artinian; Vatche (Long Beach, CA);
Brim; Terry (Redondo Beach, CA);
Matulich; Dan (Rolling Hills Estates, CA);
Murry; Roger (San Pedro, CA)
|
Assignee:
|
AlliedSignal Inc. (Morristown, NJ)
|
Appl. No.:
|
022100 |
Filed:
|
February 11, 1998 |
Current U.S. Class: |
290/52; 60/39.01 |
Intern'l Class: |
F02C 003/00 |
Field of Search: |
290/52,18
322/7,8
307/64
60/39.01,39.03
|
References Cited
U.S. Patent Documents
3024624 | Mar., 1962 | Morley et al. | 62/402.
|
3326109 | Jun., 1967 | Markham | 98/1.
|
4196773 | Apr., 1980 | Trumpler | 165/62.
|
4261416 | Apr., 1981 | Hamamoto | 165/23.
|
4494372 | Jan., 1985 | Cronin | 60/39.
|
4684081 | Aug., 1987 | Croinin | 244/58.
|
5056335 | Oct., 1991 | Renninger et al. | 62/402.
|
5114103 | May., 1992 | Coffinberry | 244/209.
|
5143329 | Sep., 1992 | Coffinberry | 244/209.
|
5145124 | Sep., 1992 | Brunskill et al. | 244/118.
|
5151022 | Sep., 1992 | Emerson et al. | 423/245.
|
5442905 | Aug., 1995 | Claeys et al. | 60/39.
|
5535967 | Jul., 1996 | Beauchamp et al. | 244/209.
|
5779196 | Jul., 1998 | Timar | 244/209.
|
5939800 | Aug., 1999 | Arinian et al. | 307/64.
|
Foreign Patent Documents |
0 888 966 A2 | Jun., 1998 | EP.
| |
Primary Examiner: Ponomarenko; Nicholas
Attorney, Agent or Firm: Zak, Jr. Esq.; William J.
Claims
We claim:
1. An air conditioning system comprising:
a first spool including a high pressure turbine;
a second spool including a low pressure turbine, an electric generator; and
a device having a non-linear load line, the low pressure turbine being
cascaded with the high pressure cooling turbine, the electric generator
generating ac power at variable frequencies when driven at variable speeds
by the low pressure turbine; and
a bypass valve for bypassing the high pressure turbine;
the bypass valve increasing the speed of the electric generator to maintain
frequency of the ac power generated by the electric generator above a
lower frequency limit, the device increasing a load on the second spool
and thereby reducing the speed of the electric generator to maintain the
frequency of the ac power below an upper frequency limit.
2. The system of claim 1, wherein the device has a speed-cubed load
characteristic such that maximum speed of the second spool is inherently
limited to maintain the frequency of the ac power below the upper limit.
3. The system of claim 1, wherein the low pressure turbine includes a
variable geometry inlet nozzle.
4. The system of claim 1, wherein the electric generator is a
voltage-regulated, variable frequency generator.
5. The system of claim 1, further comprising a mix manifold downstream the
low pressure turbine.
6. The system of claim 1, further comprising high pressure water removal
apparatus upstream the high pressure turbine, wherein the water removal
apparatus is bypassed when the high pressure turbine is bypassed.
7. The system of claim 6, wherein the first spool further includes a
compressor for boosting pressure of air entering the water removal
apparatus.
8. The system of claim 6, further comprising a reheater, downstream the
water removal apparatus and upstream the low pressure turbine, for
reheating air prior to the air entering an inlet nozzle of the low
pressure turbine.
9. The system of claim 1, further comprising means for controlling the
bypass valve to bypass the high pressure turbine when the frequency of the
ac power is approaching the lower frequency limit, whereby speed of the
electric generator is increased and the frequency of the ac power is
maintained above the lower limit.
10. The system of claim 9, further comprising a heat exchanger having a hot
side upstream the high pressure turbine, the system further comprising a
ram air door for allowing cooling air to flow through the heat exchanger,
wherein the controlling means further controls the ram air door to allow
additional air to flow through the heat exchanger while the high pressure
turbine is being bypassed.
11. An air conditioning system comprising:
a heat exchanger;
a ram air door for allowing cooling air to flow through the heat exchanger;
a first spool including a high pressure turbine for expanding bleed air
cooled in the heat exchanger;
a second spool including a fan for drawing a stream of cooling air through
the ram air door and through the heat exchanger, the second spool further
including an electric generator, and a low pressure turbine for expanding
air expanded in the high pressure turbine; and
a bypass valve coupled between a hot side outlet of the heat exchanger and
an inlet nozzle of the low pressure turbine;
the bypass valve increasing pressure at the nozzle inlet of the low
pressure turbine to increase the speed of the electric generator and
maintain frequency of ac power generated by the electric generator above a
lower limit, the fan placing a load on the second spool and increasing the
load on the second spool to reduce the speed of the second spool and
maintain frequency of the ac power below an upper limit, the ram door
increasing the flow of cooling air to the heat exchanger while the high
pressure turbine is being bypassed.
12. The system of claim 11, wherein the fan has a load line that follows a
speed-cubed load characteristic such that maximum speed of the second
spool is inherently limited.
13. The system of claim 11, wherein the inlet nozzle is a variable geometry
inlet nozzle.
14. The system of claim 11, wherein the electric generator is a
voltage-regulated, variable frequency generator.
15. The system of claim 11, further comprising high pressure water removal
apparatus intermediate the high pressure turbine and a hot side of the
heat exchanger, the water removal apparatus being bypassed when the high
pressure turbine is bypassed.
16. The system of claim 15, further comprising a compressor and secondary
heat exchanger for boosting pressure of air entering the water removal
apparatus, the compressor forming a part of the first spool.
17. The system of claim 11, further comprising means for controlling the
bypass valve and the ram air door, the controlling means controlling the
bypass valve to bypass the high pressure turbine when frequency of the ac
power is approaching the lower limit, the controlling means controlling
the ram air door to allow additional air to flow through the heat
exchanger while the high pressure turbine is being bypassed.
18. A method of operating an air conditioning system including a high
pressure turbine cascaded with a low pressure turbine, the high pressure
turbine being a part of a first spool, the low pressure turbine being a
part of a second spool, bleed air being supplied to the air conditioning
system, the method comprising the steps of:
cooling the supply of bleed air;
expanding the cooled bleed air in the high pressure turbine;
expanding air leaving the high pressure turbine in the low pressure
turbine;
bypassing the high pressure turbine when speed of the second spool
approaches a lower limit; and
adding a load on the second spool when the speed of the second spool
approaches an upper limit, the load being added according to a speed-cubed
function of the second spool speed.
19. The method of claim 18, wherein the bleed air is cooled with a heat
exchanger, and wherein the method further comprises the steps of providing
a flow of cooling fluid to the heat exchanger: and increasing the flow of
the cooling fluid while the high pressure turbine is being bypassed.
20. The method of claim 18, the low pressure turbine having a variable
geometry inlet nozzle, the variable geometry inlet nozzle having an
adjustable area, the method further comprising the step of adjusting the
nozzle area in response to variations in density of air flowing through
the low pressure turbine.
Description
BACKGROUND OF THE INVENTION
The invention relates to environmental control systems. More specifically
the invention relates to an aircraft air conditioning system including an
air cycle machine and an electric generator.
An air conditioning system for an aircraft is designed to control airflow
into the aircraft's passenger cabin as well as air temperature inside the
passenger cabin. Most aircraft air conditioning systems operate on an air
cycle refrigeration principle. Compressed air is obtained from an
intermediate compressor stage of the aircraft's main engine, cooled with
ambient air to near-ambient temperature in an air-to-air heat exchanger
and then expanded in an air cycle machine to provide a stream of cooled,
conditioned air. The conditioned air is supplied to the passenger cabin.
Although somewhat expanded, the conditioned air is still compressed in
order to pressurize the passenger cabin.
On occasion, the conditioned air might provide more cooling than necessary.
The aircraft might climb to a high altitude, or the ambient air might be
very cold. Whenever the conditioned air provides more cooling than
necessary, the cooling is reduced by a complex combination of valves and
controls.
Additionally, the engine supply pressure might sometimes be greater than
required. Whenever this occurs, the pressure is typically reduced by
throttling the compressed air. Throttling could be performed by modulating
a bleed air pressure regulator valve, a pack flow control valve, or a
bypass valve for the air cycle machine. However, throttling is a wasteful
process that causes engine fuel consumption to be greater than necessary.
Instead of wasting energy through throttling, the energy can be recovered
as disclosed by James Strang et al. in USSN 08/987,737 filed on Dec. 9,
1997 and entitled "Environmental Control System including Air Cycle
Machine and Electrical Machine." According to the Strang application,
which is assigned to the assignee of the present invention, the air cycle
machine is coupled to an electric generator. Cooling capacity and airflow
rate are reduced by driving the electric generator and extracting useful
electric power. The electric power can be used for operating electric
equipment onboard the aircraft.
However, the electric generator provides ac power at variable or "wild"
frequencies when the air cycle machine is operated at variable speeds.
Such ac power might not be usable by certain electric equipment. Electric
equipment such as fuel pumps, environmental controls and recirculation
fans might be frequency-sensitive and, therefore, require ac power having
a certain range of frequencies or even a fixed frequency. To operate such
equipment, the ac power generated by the electric generator is converted
to a usable frequency by power conditioning electronics. The power
conditioning electronics might include a rectifier for providing dc power
to the equipment running on dc power, and an inverter for chopping the dc
power to ac power having a fixed frequency.
The power conditioning electronics is expensive. For example, an inverter
might cost more than ten thousand dollars. Additionally, the power
conditioning electronics is heavy. The inverter might add ninety pounds of
weight to the aircraft.
There is a need to reduce the size and weight of the power conditioning
electronics.
SUMMARY OF THE INVENTION
Size and weight of the power conditioning electronics is reduced in an air
conditioning system according to the present invention. The air
conditioning system supplies ac power having a limited range of variable
frequencies to certain ac equipment and thereby allows the power
conditioning electronics to be reduced in size and weight. The air
conditioning system includes first and second spools. The first spool
includes a high pressure turbine. The second spool includes a low pressure
turbine, an electric generator; and controllable load-sharing means such
as a fan. The low pressure turbine is cascaded with the high pressure
cooling turbine. The electric generator generates ac power at variable
frequencies when driven at variable speeds by the low pressure turbine.
The load-sharing means is operable to increase a load on the second spool,
reduce the speed of the electric generator and thereby maintain frequency
of the ac power below an upper limit.
The air conditioning system further includes a bypass valve for bypassing
the high pressure cooling turbine. The bypass valve is controllable to
increase the speed of the electric generator and thereby maintain
frequency of the ac power generated by the electric generator above a
lower limit.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an aircraft electrical power system
according to the present invention;
FIG. 2 is a flowchart of a method of generating electrical power for an
aircraft;
FIG. 3 is a schematic diagram of an aircraft air conditioning system
including an electric generator, the generator forming a part of the
electrical power system shown in FIG. 1;
FIGS. 4 and 5 are plots of shaft speed versus time for a second spool under
different load conditions of the air conditioning system; and
FIG. 6 is a flowchart of a method of operating the air conditioning system.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an electrical power system 10 for an aircraft. The system 10
includes first and second primary power panels 12 and 14, which provide
primary power distribution to the aircraft. The first primary power panel
12 includes a first ac generator bus 16, which includes conductors such as
copper wires embedded in the hull of the aircraft and buried under panels
in the aircraft cabin. The first ac generator bus 16 receives ac power at
variable frequencies and supplies the variable frequency ac power to
frequency insensitive galley equipment 18 such as galley ovens and
chillers. The first ac generator bus 16 also supplies variable frequency
ac power to a first transformer-rectifier unit ("TRU") 20, which steps
down the ac power and converts the stepped down power to dc power. A first
static inverter 22 converts dc power from the first TRU 20 into fixed
frequency ac power. The fixed frequency ac power from the inverter 22, the
dc power from the first TRU 20 and the variable frequency ac power from
the first ac generator bus 16 are supplied to a first power management
panel 24. The first power management panel 24 includes a plurality of
power relays that can be controlled manually or automatically to provide
secondary power distribution to the aircraft. The first power management
panel 24 distributes fixed frequency ac power to loads such as hydraulic
pumps, fuel pumps, environmental controls, recirculation fans and galley
fans. Further, the first power management panel 24 distributes dc power to
loads such as communication and navigation equipment and dc
instrumentation and electronics. Still further, the first power management
panel 24 distributes variable frequency ac power to loads such as ac
lighting, gasper fans, and ice and rain protection equipment. The first
power management panel 24 also includes a plurality of circuit breakers
for line and load fault protection.
The second primary power panel 14 includes a second ac generator bus 26,
which provides variable frequency ac power to additional galley equipment
28, a second TRU 30 and second power management panel 34. A second static
inverter 32 converts dc power from the second TRU 30 into fixed frequency
ac power. The second power management panel 34 provides secondary power
distribution of variable frequency ac power from second ac generator bus
24, dc power from the second TRU 30 and fixed frequency ac power from the
second inverter 32. The second primary power panel 14, the second TRU 30,
the second inverter 32 and the second power management panel 34 can
provide system redundancy, which increases reliability of the system 10.
The ac generator buses 16 and 26 handle ac power having a limited range of
frequencies. For example, the frequency of the ac power can be between 400
Hz and 800 Hz. Powering certain equipment at variable frequencies and
other equipment at a fixed frequency allows the size and weight of the
static inverters 22 and 32 to be reduced because the static inverters 22
and 32 do not have to supply ac power to all of the equipment. For certain
aircraft, it is believed that the static inverters 22 and 32 can be
reduced in size by as much as 70 percent.
An Essential and Flight Critical Load Management Panel 36 provides ac and
dc power to selected flight instruments in the event primary power is
lost. The dc power is supplied by a battery system 38, and the ac power is
supplied by a combination of the battery system 38 and a third static
inverter 40. The third static inverter 40 converts the dc power from the
battery system 38 to fixed frequency ac power.
Primary power is supplied to either the first or second ac generator bus 16
or 26 by a generator 42, which forms a part of an aircraft air
conditioning system ("ACS"). The ACS generator 42 is operable to provide
ac power at variable frequencies, the ac power being controlled within a
limited range. Having a four-pole design and a maximum speed of 24000 rpm,
for example, the ACS generator 42 can produce ac power having a frequency
between 400 Hz and 800 Hz. The ACS generator 42 is sized to provide full
bus loads at all times to either the first or second ac generator bus 16
or 26. During normal operating conditions, the ACS generator 42 is
selectively connected to one of the first and second ac generator buses 16
and 26 by a first power relay 44 and either a second or third power relay
51 or 53.
Primary power is supplied to the other of the first and second ac generator
buses 16 and 26 by closing either a fourth of fifth power relay 50 or 52
to connect one of the aircraft's two main engine generators 46 or 48. When
a backup for the ACS generator 42 is needed, the main engine generators 46
and 48 are connected to the first and second generator buses 16 and 26,
respectively, by closing the fourth and fifth power relays 50 and 52 and
the ACS generator 42 is disconnected by opening the first power relay 44.
Typically, there will be a main engine generator 46 or 48 corresponding to
each main engine of the aircraft, and an ac generator bus 16 or 26
corresponding to each main engine generator 46 or 48. Each main engine
generator 46 and 48 is operable to provide ac power having a limited
frequency range. Having a 4-pole design and a maximum speed of 24000 rpm,
each main engine generator 46 and 48 can operate between 50% and 100% of
maximum speed and produce ac power between 400 Hz and 800 Hz.
Such an electrical power system 10 offers increased reliability due to the
additional redundancy between the main engine generators 46 and 48 and the
ACS generator 42. Such an electrical power system 10 also reduces aircraft
fuel consumption because the ACS generator 42, not a main engine generator
46 or 48, is providing electricity to one of the ac generator buses 16 or
26.
The electrical power system 10 further includes a bus power control unit
("BPCU") 54 for controlling the power relays 44, 50, 51, 52 and 53 to
connect either the ACS generator 42 or one of the main engine generators
46 and 48 to the first and second ac generator buses 16 and 26. In
addition to controlling the power relays 44, 50, 51, 52 and 53, the BPCU
54 controls the relays in the first and second power management panels 24
and 34 and the Essential and Flight Critical Load Management Panel 36. The
BPCU 54 also collects and transmits diagnostic information to the
aircraft's information system.
FIG. 2 shows control logic implemented by the BPCU 54. The ACS generator 42
is driven by the aircraft air conditioning system when, for example, the
aircraft air conditioning system is receiving bleed air from an APU or one
of the main engines 46 or 48 (block 100). The ACS generator 42 is driven
at variable speeds and, therefore, generates ac power having a range of
frequencies.
When the BPCU 54 detects that both the frequency and voltage of the ac
power are within acceptable limits (blocks 102 and 104), the BPCU 54
commands the power relays 44, 50, 51, 52 and 53 to connect the ACS
generator 42 and one of the main engine generators 46 or 48 to the first
and second ac generator buses 16 and 26 (block 106). When the BPCU 54
detects that either the frequency or the voltage goes out of range (blocks
102 or 104), the BPCU 54 commands the power relays 44, 50, 51, 52 and 53
to disconnect the ACS generator 42 and connect the main engine generators
46 and 48 to the first and second ac generator buses 16 and 26,
respectively (block 108).
FIG. 3 shows the ACS generator 42 as part of an aircraft air conditioning
system ("ACS") 56. The ACS 56 includes two spools: a first spool 58
including a compressor 60 and a high pressure turbine 62, and a second
spool 64 including a fan 66, the ACS generator 42 and a low pressure
turbine 68. The ACS 56 is typically located in the belly or tail cone of
the aircraft.
Bleed air from a compressor stage of an aircraft engine, an auxiliary power
unit or a ground cart (not shown) is supplied via a shutoff valve 70 to a
hot side of a primary air-to-air heat exchanger 72. In the primary
air-to-air heat exchanger 72, heat of compression is removed from the
bleed air and dumped to ambient.
Hot side outlet air of the primary heat exchanger 72 is compressed by the
compressor 60 and supplied to a hot side of a secondary heat exchanger 74.
In the secondary air-to-air heat exchanger 74, heat of compression is
removed and dumped to ambient. The compressor 60 provides an air cycle
lift, which allows the primary heat exchanger 72 to be smaller.
High pressure water separation is then performed. Air leaving the hot side
of the secondary air-to-air heat exchanger 74 is supplied to a hot side of
a reheater 76, where additional heat is removed. Air leaving the hot side
of the reheater 76 is supplied to a hot side of a condenser 78, which
causes moisture entrained in the air to form condensate. Higher pressure
resulting from the compressor 60 enhances the formation of condensate and
reduces the presence of vapor in the air.
Air leaving the condenser 78 is supplied to a water extractor 80. Inside
the water extractor 80, a vaned swirl section centrifuges the condensate
against a duct wall, causing the condensate to be trapped in an annular
space formed by a duct wall liner that begins downstream of the vaned
swirl section. The duct wall liner isolates dehumidified air from
moisture-laden air in the annular space. The condensate trapped in the
annular space is collected by a sump that is several duct diameters
downstream of the vanes. The condensate collected by the sump is sprayed
at a ram air inlet of the secondary heat exchanger 74.
Dehumidified air leaving the water extractor 80 is supplied to a cold side
of the reheater 76 and reheated. Reheating puts more energy into the
dehumidified air, which allows for greater cooling in the high pressure
turbine 62. Additionally, reheating increases turbine reliability by
minimizing the formation of ice on the wheel of the high pressure turbine
62.
Air leaving the hot side of the reheater 76 is expanded in the high
pressure turbine 62 and cooled to a temperature between 0.degree. F. and
50.degree. F. Shaft power resulting from the expansion in the high
pressure turbine 62 is used to drive the compressor 60. Air leaving the
high pressure turbine 62 is supplied to a cold side of the condenser 78,
where additional energy from a roughly 50.degree. F. rise in temperature
is put into the air. Such reheating increases cooling and reliability of
the low pressure turbine 68. Air leaving the cold side of the condenser 78
is expanded in the second cooling turbine 68 and cooled to sub-freezing
temperatures.
The sub-freezing air from the low pressure turbine 68 is supplied to a mix
manifold 82. Inside the mix manifold 82, the subfreezing air is mixed with
air from the aircraft cabin. Cool, conditioned air leaving the mix
manifold 82 is supplied to the aircraft cabin.
The low pressure turbine 68 has a variable geometry inlet nozzle 84, which
optimizes airflow and power extraction. The area of the nozzle 84 is
adjustable to control the flow of cooled, conditioned air into the cabin
as the cabin pressure and bleed air pressure change. When pressure is
increased, air density becomes higher and, therefore, less volumetric flow
is needed to achieve the same mass flow through the cabin. Therefore, area
of the nozzle 84 is decreased. Conversely, the area of the nozzle 84 is
increased when pressure is decreased. A flow sensor 85 provides feedback
to the nozzle 84 for the adjusting the nozzle area.
Shaft power resulting from the expansion in the low pressure turbine 68 is
used to drive the fan 66 and the ACS generator 42. The ACS generator 42 is
a voltage-regulated, variable frequency generator such as a traditional
wound rotor rotating rectifier machine that supplies constant voltage at
variable speed. Such a machine is an industry standard for aircraft power
generation. The wound rotor is mounted directly to a common shaft 86 and
driven at shaft speeds, without the need for a gearbox. In the
alternative, the ACS generator 42 could be a Rice machine or a homopolar
machine.
When ac power is extracted from the ACS generator 42 (for example, by
turning on a piece of electrical equipment), a load is placed on the shaft
86. The load is transmitted by the shaft 86 to the fan 66 and the lower
pressure turbine 68.
The ACS generator 42 is driven by the low pressure cooling turbine 68 at
variable speeds because generator load and bleed flow conditions change
during the flight of the aircraft. For example, the ACS generator 42 might
be operated between 50% and 100% of maximum speed. Consequently, the ACS
generator 42 generates ac power having a variable frequency and constant
voltage. The variable frequency, constant voltage ac power is extracted
from the ACS generator 42 and supplied directly to either the first or
second ac generator bus 16 or 26.
The fan 66 draws ambient air through a ram air door 88, into a supply
plenum 90 and through the heat exchangers 72 and 74. Heat is carried away
from the heat exchangers 72 and 74 and dumped to ambient. A check valve
(not shown) allows air to flow around the fan 66, if necessary. The fan 66
provides the sole source of cooling air for the heat exchangers 72 and 74
while the aircraft is on the ground. Therefore, the fan 66 is designed for
maximum efficiency at a minimum operating speed (e.g., 50% of maximum
speed).
The fan 66 has a load line that follows a speed-cubed characteristic. When
the shaft speed of the second spool 64 increases above a nominal design
speed, the fan load increases as a non-linear function of the shaft speed.
The speed-cubed characteristic of the fan 66 is advantageously used to
create a balanced load-sharing between the ACS generator 42 and the fan
66. While energy is being extracted from the ACS generator 42, a generator
load is placed on the shaft 86. When the load is shed (for example, by
turning off a galley oven), speed of the shaft 86 begins to increase.
However, the increase in shaft speed causes the fan load to increase
non-linearly and ultimately limit the shaft speed. The shaft speed is
inherently maintained below an upper limit without the need for a speed
governor or any other active speed control. When the ACS generator load is
reapplied, the shaft speed and fan load are decreased. Thus, an upper
limit can be set by proper design of the fan 66.
Speed of the shaft 86 and, therefore, frequency of the ac power generated
by the ACS generator 42 can be maintained above a lower limit by a bypass
valve 92. The bypass valve 92 is coupled between a hot side outlet of the
primary heat exchanger 72 and an inlet of the low pressure turbine 68.
When the shaft speed is approaching the lower limit, the bypass valve 92 is
opened to increase the pressure to the low pressure turbine 68 and thereby
increase the speed of the ACS generator 42 above the lower limit. Since
the high pressure turbine 62 is bypassed, the benefit of cascading the
high pressure cooling turbine 62 (i.e., greater air cycle lift) is lost.
However, the ram air door 88 is opened to allow for more cooling by the
heat exchangers 72 and 74. The increase in heat transfer to ram air lowers
the temperature of the bleed air leaving the heat exchangers 72 and 74
and, therefore, partially compensates for the loss of cycle lift. The
variable nozzle 84 is opened somewhat to allow for a constant supply of
air to the cabin.
When shaft speed is approaching the upper limit, the pressure supplied to
the low pressure power turbine 68 is lowered by closing down the bypass
valve 92 and diverting additional air through the first spool compressor
60 and high pressure turbine 62. Consequently, compressor work increases
the air cycle lift. The ram air door 88 is closed down to limit the flow
of cooling air across the heat exchangers 72 and 74, as the cycle lift
would otherwise make the air leaving the mix manifold 82 too cold. The
variable nozzle 84 is opened somewhat to allow for a constant supply of
air to the cabin.
Reference is now made to FIG. 4, which shows an example of shaft speed of
the second spool 64 being kept below an overspeed limit. Assume that the
second spool 64 is operating at a nominal speed with the galley oven
turned on. That is, the second spool 64 is operating at nominal speed
while the ACS generator 42 is placing a large load on the shaft 86. When
the galley oven is turned off, the large ACS generator load is shed and
the speed of the second spool 64 begins to increase. However, the speed of
the fan 66 also begins to increase. Additionally, the fan load on the
shaft 86 begins to increase as a function of speed-cubed. As the fan load
is increased, the overspeed limit is approached, but not exceeded.
Reference is now made to FIG. 5, which shows an example of shaft speed of
the second spool 64 being kept above an underspeed limit. Assume that the
second spool 64 is operating at a nominal speed with the galley oven
turned off. When the galley oven is turned on, the ACS generator 42 places
a large load on the shaft 86, causing the shaft speed begins to decrease.
The bypass valve 92 is opened fully to provide bottoming control and keep
the shaft speed of the second spool 64 above the underspeed limit. After
the bypass valve 92 is opened, the shaft speed begins to increase. The
underspeed limit is approached, but never reached. Cabin temperatures are
kept at comfortable levels by opening the ram air door 88.
Returning to FIG. 3, the ACS 56 also includes a temperature control valve
94, which allows a portion of the bleed air to bypass the primary heat
exchanger 72 on cold days or in full heating mode. The temperature control
valve 94 allows the temperature of the air leaving the mix manifold 82 to
be increased. Additionally, the temperature control valve 94 provides
protection against temperatures dropping to subfreezing levels in the
water extractor 80.
A controller 96 receives signals from sensors 98 such as a shaft speed
sensor and temperature sensors indicating temperatures of air entering the
water extractor 80, air leaving the low pressure turbine 68 and air
leaving the compressor 60. The shaft speed sensor signal allows for
control of the shaft speed and, therefore, ac power frequency. The
temperature sensor signals allow for basic temperature control as well as
protection against system icing and compressor overtemperature. In
response to the sensor signals, the controller 96 controls the ram air
door 88, the bypass valve 92 and the temperature control valve 94. The
controller 96 also controls the shutoff valve 70.
FIG. 6 illustrates the air cycle of the ACS 56. Bleed air is supplied to
the primary heat exchanger 72 (block 200) and cooled (block 202). The
cooled bleed air is compressed by the compressor 60 (block 204) and cooled
again in the secondary heat exchanger 74 (block 206). High pressure water
extraction is performed on the air leaving the secondary heat exchanger 74
(block 208), and the resulting dehumidified air is expanded and cooled in
the high pressure turbine 62 (block 210). Heat is transferred to the
condenser 78 (block 212), giving a cycle lift. Air leaving the condenser
78 is expanded and further cooled to subfreezing temperatures in the low
pressure turbine 68 (block 214). The subfreezing air is supplied to the
mix manifold 82, and a mixture of the subfreezing air and cabin air is
supplied to the aircraft cabin. The cycle continues.
Shaft power is created by the expansion of air in the low pressure turbine
68. The shaft power, in turn, causes the ACS generator 42 to generate ac
power (block 218). Varying shaft speeds will result in ac power having
varying frequencies. When the shaft speed of the second spool 64 and,
therefore, the frequency of the ac power is going too low (blocks 220 and
222), the bypass valve is opened (block 224). Since the high pressure
turbine 62 is being bypassed, the ram air door 88 is opened to increase
cooling (block 226). If the frequency of the ac power is still too low, a
load on the ACS generator 42 can be shed by turning off electrical
equipment (block 228). Although the underspeed limit is approached, it is
never reached. The ACS generator 42 continues generating ac power.
When the shaft speed of the second spool 64 and, therefore, the frequency
of the ac power is going too high (blocks 220 and 22), the fan load
increases as a function of speed-cubed (block 230). The overspeed limit is
approached but, due to the rapidly increasing fan load, it is never
reached. The ACS generator 42 continues generating ac power.
Actual control of the bypass valve 92, temperature control valve 94 and ram
air door 88 will depend upon flight and electrical load conditions. In
situations where maximum refrigeration is required, the bypass valve 92
and the ram air door 88 can be fully opened by the controller 96. In
situations where the temperature of the ambient air is cool and
refrigeration is not required, the ACS 56 will deliver electricity and
warmer air, as required. The controller 96 will open the temperature
control valve 94 or open the bypass valve 92.
In a large commercial aircraft, cooling is more difficult to perform during
ground operation because the fan 66 supplies all of the cooling air.
During flight, however, ram air pressure provides all of the cooling flow
needed, which allows the high pressure turbine 62 to be bypassed. At
altitudes below the cloud tops, where ambient moisture could be present,
all of the cycle air passes through the first spool 58 and the high
pressure water extraction loop.
The ACS 56 can be run on the ground on a hot day when the main engines are
off and still meet certain cooling requirements. This is called an
Auxiliary Power Unit ("APU") condition. The APU supplies refrigerated air
to the cabin and electricity to the aircraft. The APU shaft load shifts to
the compressor which must deliver higher bleed pressures in order for the
ACS 56 to provide both air conditioning and electricity. For electric
power generation only, the load compressor bleed pressure is dropped to an
unloaded condition using inlet guide vanes. The ACS 56 operates in a high
pressure spool bypass to deliver ambient temperature ventilation air to
the cabin.
Thus disclosed is an aircraft air conditioning system that generates ac
power by recovering energy that would otherwise be wasted. Fuel is saved,
and the cost of operating the aircraft is lowered.
The ACS 56 can maintain shaft speed and, therefore, frequency of the ac
power within upper and lower limits. Consequently, the ac power can be
used to operate variable frequency equipment onboard the aircraft, which
allows for the size and weight of the inverters to be reduced.
Additionally, the ACS 56 includes cascaded high and low pressure turbines,
which results in a more efficient thermodynamic cycle and allows the air
to be cooled to subfreezing temperatures. Consequently, less air is used
to cool the aircraft cabin.
The ACS 56 is used advantageously in the electrical power system 10 above,
but it is not limited to such use. The ACS 56 is especially useful for any
aircraft having on-board electrical equipment that can be operated at
variable frequencies. Thus, the ACS is especially useful for commercial
aircraft. However, the ACS 10 can also be useful for regional, corporate
and military aircraft.
The invention is not limited to the specific embodiments described above.
For example, the ACS 56 could use a water separator instead of the water
extractor 80. Any device following a non-linear load line could be used in
place of the fan 66. One such device is a turbocompresssor, which also has
a speed-cubed load characteristic. The compressor 60 in the first spool 58
could be replaced by a fan, which would allow the secondary heat exchanger
74 to be eliminated.
These considerations, and other considerations including the size of the
ACS generator 42, the size of the first and second spools 58 and 64, the
sizes of the reheater 76 and the heat exchanger 72 and 74, are all
dependent upon the application for which the ACS 56 is intended.
Therefore, the invention is not limited to the specific embodiments above.
Instead, the invention is limited only by the claims that follow.
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